1. Field of the Invention
The present invention relates to a non-impact dot printer and method for printing halftone or continuous tone information and the like with small pixels (dots).
2. Description of the Prior Art
Various non-impact recording or printer heads for use as dot printers are known. Examples of such heads include an electrostatic printer head, an LED (light emitting diode) array printer head, ink-jet printer head, thermal printer head, etc. While the invention will find utility with regard to non-impact printers in general, discussion will be made herein with regard to LED printer heads with which the invention is particularly suited.
In U.S. Pat. No. 4,750,010 an LED printer is described in which a series of LED's arranged in a straight line are selectively activatable for brief periods to form dot-like images on a recording surface. One feature of this printer is that the signal pulses to all the selected LED's causing same to be activated during any one PEL period (picture element period or dot line period) are symmetrical about a center line. The term used to describe this arrangement of signals is "center pulse width modulated." The reasons for arranging such signals in this manner are described in the patent. In one embodiment of a printer head described in this patent, grey scale recording is achieved by having a digital comparator associated with each LED. At one input to the comparator, there is provided the data in the form of a plurality of digital data bits representing an exposure on-time for that LED for that PEL period. At a second input to the comparator, an input from an up/down counter is provided that is rapidly changing in accordance with signals emitted from a high speed clock. As the counter output value decreases towards zero at some point in time a match is sensed by the comparator between the two inputs of the comparator (in accordance with its operating criterion). The LED then turns on and remains on for a very brief duration until a subsequent sensing of a match by the comparator occurs during a count-up phase of the counter. The above patent notes that the data may be adjusted to provide also for exposure balancing of other LED's. This is desirable where non-uniformity in illumination from LED to LED on the printer head may be expected. While the patent notes that a programmable clock may be employed to adjust the time period for each clock pulse, the clock periods will nevertheless be uniform after any such adjustment.
A problem with the above is that it does not efficiently cover as much exposure space as might be desirable for grey level recording and taking into account the use of exposure time to control uniformity between LED to LED. Assuming one employs six data bits to define each pixel, 63 levels of grey may be recorded (not including white, i.e., no activation.) However, these 63 levels must be recordable within a time period comprised of at most 126 uniform clock periods plus a minimum established time, T.sub.MIN. Where the clock periods are of relatively short duration, the 63 levels of grey may not provide the desirable range of grey levels. On the other hand, where the clock periods are of relatively longer duration, adequate extremes of grey may be recorded but fine differences in grey level recording particularly desired at relatively lower grey values will be lost.
In commonly assigned U.S. application Ser. No. 290,002, filed Dec. 27, 1988 in the names of Pham et al, a non-impact printer is described which provides improved grey level recording. Briefly, this is accomplished by providing a non-linear clock for calculating exposure times. The clock provides relatively short duration pulses used to control exposure times in the "toe" area of the density curve (reflection density vs. exposure time) and relatively larger duration pulses to control exposure times in the "knee" area of this curve. While this approach works well, still further improvements in control of exposures at the toe area would be desirable. While improvements may be obtained using clocks having significantly higher frequencies, the increased costs of same makes recourse thereto undesirable.
It is therefore an object of the invention to a non-impact grey level printer and method for printing which provides for improved grey level recording.